The differentiation of Leishmania promastigotes from a non- infective to an infective or metacyclic stage is accompanied by changes in surface carbohydrates as detected by the lectin PNA. This change is a consequence of the sialation of a cell surface and released glycolipid, which can be identified by the monoclonal antibody 3F12, which is specific for metacyclic promastigotes of L. major. The expression of the modified form of the glycolipid appears to influence the type of receptors on the macrophage which the organisms use the attachment and uptake. Metacyclic promastigotes can be shown to use CR1 receptors, because their attachment can be inhibited by mAb against these receptors. Non-infective promastigotes from log phase cultures cannot be shown to utilize C3 receptors. Opsonization of promastigotes with C3 enhances their uptake and promotes their survival, indicating that the choice of receptors for attachment influences subsequent intracellular survival. The molecular nature of these receptor ligand interactions and consequences will be the basis of further studies in terms of the role of the glycolipid and in particular the sialic acid, the role of other cellular receptors for carbohydrates, and the role of complement activation and C3b deposition in these events.